Humans use visual features of the environment (landmarks) to allow them to navigate: finding new routes and following those learnt previously. An important subset oflandmarks is objects. Despite knowing this, we still do not have a complete picture of how landmarks are used during human navigation. The work presented here assesses how the location of an object along a route influences its use as a landmark. Thus, the current work provides a theory of landmark use in human navigation, defining first how objects become encoded as landmarks and secondly how those objects are used to guide directional decisions. The first five experiments test recognition of objects placed along a learnt route, to discover how objects become landmarks. These experiments reveal an important and novel finding of a difference in recognition for objects placed at decision points at which a turn occurs compared to decision points at which no turn occurs. By including a temporary turning motion at decision points where no permanent turn occurs, it is shown that the presence of a turn is the key factor driving the difference in object recognition at the two decision point locations. The change in optic flow that occurs at such turns is put forward as the basis for this difference. The change in optic flow triggers the encoding of objects at turns as key landmarks. In the final five experiments, participants make directional decisions when presented with images of decision point junctions at which a turn occurs, after having learnt the route. These experiments were designed to investigate how people use landmark objects while retracing a learnt route. These experiments demonstrated that participants use the objects as beacons (selecting the directions based on the location of the object at the junction). This was shown by changing the location of half of the obj ects from one side of a junction between the learning and testing phases of the experiments. During the trials where the obj ects had changed position, participants made systematic errors due to the change. Thus, this thesis provides a theory of the use of landmarks in humans and increases our understanding of human navigation.